Medical instruments and medical materials are used in the situations where body fluid such as blood and urine may adhere to them, so that they should be made of plastics as a disposable article for single use, and their disposal needs to be carried out carefully by incineration or the like for the prevention of viral or bacterial infection. The use of such disposable instruments and materials, however, will increase waste, leading to an increase in carbon dioxide emitted by the incineration of waste.
Biodegradable resins are decomposed at the end of the product life cycle by microorganisms present in nature into carbon dioxide and water, imposing least burden on the environment. The biodegradable resins as such are expected to find wide application in various fields as a material for agriculture, a material for civil engineering and construction, or any other industrial material. Among others, those biodegradable resins which are made from plant-derived raw materials are also receiving attention from the viewpoint of global warming suppression for the reason that the carbon dioxide emitted during the disposal of such resins is to be absorbed by growing plants and, consequently, the total amount of carbon dioxide is not changed (carbon-neutral status).
Examples of the plant-derived biodegradable resins include polybutylene succinate as a flexible example, polylactic acid and poly(3-hydroxyalkanoate) as a relatively stiff example, as well as copolymers, blends and polymer alloys thereof.
Because of their reduced burden on the environment, biodegradable resins are very useful as a resin for use in the medical instruments and medical materials which are generally thrown away after being used one time.
Unfortunately, biodegradable resins are low in heat resistance, mechanical strength and moldability as compared with such general-purpose resins as polyethylene and polypropylene. It is necessary for an extensive, full-scale use of biodegradable resins to improve their physical properties through resin design, by addition of modifiers, and so forth.
Polybutylene succinate resins are the polyethylene-like resins of a flexible nature whose excellent impact resistance makes them suitable for parts of medical instruments. The polybutylene succinate resins can be made stiffer by mixing therein a polylactic acid resin or poly(3-hydroxyalkanoate) resin, so that the design for materials of a polybutylene succinate resin is easy to make in accordance with the intended use of the resin. It, however, is not possible to blend polylactic acid into a polybutylene succinate resin for the modification of the latter because the reduction in impact resistance due to the irradiation with ionizing radiation is an issue to be addressed for the polybutylene succinate resins also, and a considerable reduction occurs particularly in a resin having polylactic acid blended therein.
In addition, the polybutylene succinate resins have a melting point of about 110° C., which is lower than 115° C., the temperature as defined for the autoclave sterilization generally applied to medical instruments and medical materials. On a medical instrument and a medical material each composed of a polybutylene succinate resin alone or a mixture of a polybutylene succinate resin with a polylactic acid resin or poly(3-hydroxyalkanoate) resin, accordingly, the autoclave sterilization cannot be conducted due to a possible thermal distortion.
Ionizing radiation, as enabling sterilization approximately at normal temperatures, is suitable for the sterilization of less heat-resistant resins, but inappropriate to the sterilization of a medical instrument including a liquid such as an injection vessel, so that a heat-sterilizable resin is desired for such an instrument.
Crosslinking by means of the irradiation with ionizing radiation is disclosed as a technique for improving the heat resistance of polybutylene succinate (see Non-Patent Document 1, for instance). The ionizing radiation as used has an intensity of 210 kGy, so high an intensity as to inspire fears that the resin might be deteriorated.
Under the circumstances where the sterilization by ionizing radiation is frequently applied to medical instruments owing to its convenience, biodegradable resins are not used for medical instruments because the resins are considerably reduced in strength and impact resistance by the irradiation with ionizing radiation as compared with the general-purpose resins.
In a disclosed method of producing an ionizing radiation-sterilizable molded article of a biodegradable resin, a radiation crosslinker is added to a biodegradable resin, then the resin is sterilized by ionizing radiation (see Patent Document 1, for instance). During the irradiation with ionizing radiation, the radiation dose needs to be controlled in order to adjust the strength and the impact resistance of the resin as irradiated. Also disclosed is the technique for improving the hydrolysis resistance of a biodegradable resin that includes addition of polycarbodiimide as a terminal blocking agent (see Patent Document 2, for instance). It is not established yet whether or not the addition of polycarbodiimide to a biodegradable resin allows an inhibited reduction in strength and impact resistance during the sterilization by ionizing radiation.    Non-Patent Document 1: J. Macromol. Sci.—Pure Appl. Chem., A38(9), 961-971 (2001).    Patent Document 1: JP 2004-204195 A    Patent Document 2: JP 3776578 B